Using high frequency alternating electric current to melt certain metals is well known in the art. One type of furnace used in such procedures is the coreless induction furnace. See FIG. 1. The coreless induction furnace is characterized by a water cooled helical copper coil C that carries alternating current and is encased in ceramic grout G. This coil is wound around the outside of a cylindrical crucible R which holds a solid metal charge M. When the high frequency current flows within the coil, its resulting magnetic field induces an electric current in the metal charge. The metal charge's inherent resistance to that current causes the temperature of the metal charge to rise, eventually rendering it molten.
The conventional crucible of a coreless induction furnace typically comprises a protective refractory layer set in place by ramming dry refractory grain mixes (such as mullite-bonded alumina, spinel-bonded alumina and spinel-bonded magnesia) into place between a metal former and the ceramic grout. When rammed into place, these refractory mixes typically have a porosity of about 18% and a mean pore radius of about 10 um. If the rammed refractory remained in this state, its high porosity and large pore size would provide little or no resistance to molten metal. However, during furnace start-up (i.e., shortly before and during metal former meltdown), the rising temperature of the metal former gives off enough heat to cause the refractory grains at the metal/grain interface to bond together, resulting in a more dense ceramic surface layer (or "skin") having a porosity of about 10% and a pore size of about 8 microns (um) backed by unbonded refractory grains. This denser surface layer provides resistance against metal migration into the refractory. In addition, the porous nature of the remaining unbonded refractory grains not only provides a thermal expansion cushion for the denser surface layer when the denser surface layer contacts molten metal, it also provides an additional barrier against further molten metal migration (should the denser surface layer crack or be otherwise compromised) by self-bonding when exposed to a molten metal front.
The rammed refractory design described above has been somewhat successful in preventing metal migration in conventional coreless induction furnaces. For example, a 22 inch diameter coreless induction furnace processing gray iron for about 6 months at 1520.degree. C. typically shows iron migration extending about one-quarter of the way into its four inch refractory layer. When the migration penetrates about half of the refractory layer, the refractory layer is typically replaced.
Although this conventional furnace has been somewhat successful in retarding leaks from conventional coreless induction furnaces, the performance requirements of induction furnaces are now becoming increasingly more ambitious. In particular, the furnaces are operating at higher frequencies (1000 Hz vs 60 Hz) and higher temperatures (at least 2950.degree. F. vs 2700.degree. F.), resulting in more severe conditions. Accordingly, these new operating conditions require reconsideration of the protection provided by the conventional refractory layer of coreless induction furnaces.
U.S. Pat. No. 5,134,629 discloses a "core and coil" induction furnace whose refractory barrier includes a flame-sprayed ceramic coating. U.S. Pat. No. 3,914,527 discloses a "core and coil" induction furnace whose refractory barrier includes fused silica. EPO Patent Publication 0 069 094 A1 discloses a "core and coil" induction furnace whose refractory barrier includes a sprayed, cast or brushed-on refractory layer. However, merely borrowing refractory designs from these conventional "core and coil" induction furnaces is considered to be of little value, since the "core and coil" induction furnace operates under relatively mild conditions (i.e, frequencies of 60 Hz and temperatures of 2750.degree. F.) in comparison to the newer, high frequency coreless induction furnaces.
One particular concern related to the new operating conditions in a coreless furnace is that the higher frequencies generate a magnetic field which is not only stronger, but is also situated closer to the protective refractory layer. Accordingly, when molten metal penetrates the refractory, it moves towards the coil and in so doing enters into an even stronger portion of the magnetic field. The stronger magnetic field heats the stray metal to even higher temperatures, thereby facilitating its migration through the refractory. In a worse case scenario, the metal gets hotter and migrates farther until it finally reaches the cooling water surrounding the coil. When the 2750.degree. F. metal contacts the water, it dissociates the water into hydrogen and oxygen which upon recombination cause violent and catastrophic explosion.
Accordingly, there is a need for a coreless induction furnace having a protective refractory layer which will resist molten metal penetration even under modern operating conditions.